1. Field of the Invention
This invention relates to an optical disk having a synchronous mark for synchronizing detection and a data field for recording data on each track.
2. Description of the Related Background Art
In an optical disk having a recording format for a sampled servo system, a synchronous pattern provided with two marks (pits) is formed on the track of each servo field in order to obtain synchronizing signals for reading recorded data. The distance between the two marks is made longer than the maximum inversion interval which is the maximum distance between the marks in the marked portion indicating the data. For example, the distance is longer than the maximum inversion interval by 1 T (where the T is one pit interval). When the disk is played, a reading operation is performed, at first, while the servo system is being operated by asynchronous master clock pulses, thus detecting the synchronous pattern from the read signal. After the detection of the synchronous pattern, a clock mark is detected from the read signal. In synchronism with the detection of the clock mark, a clock pulse is generated in a PLL circuit (phase-lock-loop circuit). Clock pulses reproduced for reading the data are obtained from the PLL circuit when the PLL circuit arrives at its locked state.
In the case of an optical disk of a CAV (Constant Angular Velocity) type, where the rotational speed is constant, it is easy to detect the synchronous pattern from its length after the rotational speed has once reached the constant speed. As a result, it is possible to reproduce the clock pulses from the read signal as described above.
However, in the case of the optical disk of the CLV type, where its linear velocity is constant, the rotational speed of the spindle is not constant as the disk is driven to rotate. Therefore, it is not easy to detect the synchronous pattern merely from its length. The portion of the maximum inversion interval must be detected by counting the master clock pulses to servo control the spindle so that such portion should be provided with a predetermined temporal length rather than linear length. Thus, if the maximum inversion interval in the data is given as kT, and the length of the synchronous pattern as longer, is one pit interval longer as stated above, i.e.(k+1) T, then the length of the synchronous pattern becomes (k+1) T.+-.0.5 T. The 0.5 T is included to factor in the error due to the asynchronous master clock pulses when it is intended to detect the synchronous pattern by using master clock pulses having double frequency. If, in addition the rotational error of the spindle is 0.5 T or more, then the counted value by the master clock pulses of the synchronous pattern could possibly yield a value kT. Therefore, since kt also corresponds to the length of the maximum inversion interval in the case of the optical disk of the CLV type, a problem arises in that it is not easy to detect the synchronous pattern because it is hard to distinguish it from the maximum inversion interval in the data.